* {
    margin: 0;
    padding: 0;

}

.box {
    position: relative;
    width: 600px;
    height: 400px;
    margin: 40px auto 0;
    box-shadow: 0 0 10px 5px rgba(153, 153, 153, 0.6);
    border: 1px solid #999;
}

#list {
    position: absolute;
    width: 600px;
    height: 400px;
}

#list img,#prev,#next,#btn {
    position: absolute;
}

#list img:not(:first-child){
    display: none;
}

#prev,#next {
    width: 40px;
    height: 70px;
    background: url("../images/icon-slides.png") no-repeat;
    border: none;
    outline: none;
    cursor: pointer;
    z-index: 100;
    top: 50%;
    transform: translateY(-50%);
}

#prev {
    background-position-x: -86px;
}

#next {
    right: 0;
    background-position-x: -125px;
}

#btn span {
    cursor: pointer;
    width: 20px;
    height: 20px;
    display: inline-block;
    border-radius: 50%;
    border: 1px solid #eee;
    background-color: rgba(153, 153, 153, 0.5);
}

#btn {
    bottom: 20px;
    /* 先相对自己左移一半 */
    transform: translateX(-50%);
    /* 后让自己移动父元素一半 */
    margin-left: 50%;
}

#btn .select {
    background-color: #55242488;
}